The invention relates to solvent-free coating compositions which can be cured with low emissions, and especially printing inks, which comprise, as binders, monomer-free saturated and/or unsaturated polyester resins containing dihydrodicyclopentadiene and/or oligodihydrodicyclopentadiene structural units, and to their use, in particular, for metal containers, for example cans.
Unsaturated polyester resins having dicyclopentadiene structural units as constituents of customary UP resin systems are known. In general, such UP resin systems contain a monomeric reactive diluent such as, in particular, styrene, which give rise to emission problems in the course of processing of the UP resins.
Also known are printing inks, especially for metal containers, consisting of binder, solvents, pigments and auxiliaries. Examples of known binder systems are alkyd resins, which are crosslinked with melamine- or urea-formaldehyde resins and which are employed in printing inks or printing materials with typical solvent contents of up to 50%.
Known alternatives to the solvent-containing systems are, as relatively low-emission systems, powder coatings, aqueous coating systems and liquid, (UV-)radiation-curable coating materials.
Powder coatings have the disadvantage that the requirements for good storage stability, especially for blocking resistance, and for good flow on melting of the powder coating are often at odds with one another.
Disadvantageous features of the aqueous systems are that the evaporation of the water is expensive, especially energy-intensive, and that the water usually has a damaging effect on the substrates to be coated.
The liquid, (UV-)radiation-curing systems, in which in general all of the components of the coating material remain in the resulting coating, do indeed have good leveling and good coating properties, but on the other hand the toxic potential of the reactive diluents and/or of the photoinitiators employed, and the inhibition of curing on the coating surface by means of atmospheric oxygen, are often of great disadvantage.
DE-A-27 08 846 describes the partial replacement of the reactive diluent styrene in the customary UP resin systems by specific polyesters having dicyclopentadiene structural units, and the use of such UP resin systems as materials. The residue of the reactive diluent, styrene, which remains is objectionable for the application of such resin systems, in particular, as printing inks which can be cured and processed with low emissions, on ecological grounds and on the grounds of pollutant contamination at the workplace.
DE-A-31 07 450 comprises unsaturated polyesters with dicyclopentadiene oligomers as end groups, which are used as solutions in ethylenically unsaturated monomers in order to produce moldings and coatings. The ethylenically unsaturated monomers employed as reactive diluents are generally problematic as a result of their high vapor pressure at room temperature and processing temperature and of the emission problems associated with this.
EP-A-0 101 585 describes unsaturated polyester resins which are modified by the addition of cyclopentadiene onto the double bonds of the unsaturated units of the polyester and are then dissolved in vinyl monomers as reactive diluent.
Again, the use of vinyl monomers as reactive diluent causes problems from the ecological and toxicological standpoint.